clk: ti: am33xx: Keep am3 l3 main clock always on for genpd

In order for suspend and resume to work with genpd on am3, we must keep
l3 main clock always on. Otherwise prm_omap driver will shut down the l3
main clock on suspend when simple-pm-bus and GENPD_FLAG_PM_CLK are used.
Note that we already keep the l3 main clock always on with the legacy
platform code.

Later on we may want to start managing the l3 main clock with a dedicated
interconnect driver instead of using simple-pm-bus and GENPD_FLAG_PM_CLK.

Cc: linux-clk@vger.kernel.org
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>